This invention provides an improved method and apparatus for controlling temperatures within streams of molten material in furnace assemblies. In particular, this invention addresses the problem of thermal lags within the material and variations in the temperature of material entering the various sections of such molten stream. While this invention has application to many different processes, for the purpose of illustration, this control system will be described with respect to its application in the glass fiber production process.
The furnace of the glass fiber production process traditionally melts glass batch or cullet in a melting and refining tank. From the melting and refining tank, the molten glass flows into the forehearth section of the furnace. The forehearth section is comprised of numerous channels which supply streams of molten glass to one or more producing devices such as bushings or spinners. These devices attenuate the glass into staple fibers, continuous strands, or other products through conventional processes.
The quality of the glass fibers attenuated in such a process is highly dependent on proper control of the temperature of the glass stream as it passes through the channels of the forehearth to the producing devices. These forehearth channels have traditionally been divided into a number of successive heating zones, each with individual temperature sensing and heat control equipment. The molten glass enters the forehearth section at a temperature of roughly 2500.degree. F., and should generally be delivered to the producing devices at temperatures near 2300.degree. F. for optimal production efficiency. The degree of difficulty involved in this task is further compounded in that the geometric configuration of traditional forehearth assemblies dictate that different streams of glass must often traverse greatly different distances in the course of their normal flow from the melter and refiner exit, through the successive forehearth zones, to their respective producing assembly.
One traditional approach to forehearth temperature control assigns a single temperature regulator to each forehearth zone. This regulator responds to changes in the temperature of a single temperature transducer in the hot gases above the stream of molten glass in the forehearth channel by controlling the fuel-air supply to the burners associated with that particular forehearth section. In general, it has not proved difficult to control such an atmospheric temperature which is sensed by a single transducer in the hot exhaust gases above its associated forehearth zone. The major drawback in such a system is that it is not well adapted to control the temperatures within the molten stream of glass being conveyed to the producing devices of the forehearth. Instead, this system focuses on the control of temperatures in the hot exhaust gases which is seldom effective in adequately controlling the characteristics of the glass which affect production efficiency.
A more recent control method is one such as that disclosed in the patent to Griem Jr., U.S. Pat. No. 3,506,427, issued Apr. 14, 1970. This patent discloses a technique for compensating temperature control in forehearth zones for masses of unmelted glass batch passing through the channels of the forehearth. In this technique, each forehearth zone's temperature controller responds directly to a temperature measurement within the glass of that particular zone by adjusting the heat input of said zone to achieve a desired glass temperature. In order to dampen response of such controllers to cool masses of glass passing through the individual forehearth zones, Griem suggests that such masses be detected upon entrance to the forehearth and effectively tracked in their journey through the forehearth channels, compensating each zone's controller as its associated temperature transducer becomes affected by the cool mass. One difficulty with this technique is that this compensation consists primarily of deactivation of the control mechanism for the particular forehearth zone. Another difficulty is that no consideration has been devoted to the thermal lags which occur in the stream of molten glass between the heating mechanism and the temperature transducer.
Another system for controlling the temperature in molten glass is in U.S. Pat. No. 4,028,083 which discloses a furnace, which includes a melting and refining tank and a forehearth, is divided into a plurality of zones or regions. Each of the zones is provided with means for sensing temperature within the zone and a means for heating the zone. Means is also provided for measuring the individual heat input into the furnace of the heating means in each of the zones. When changes in temperatures are required, the temperatures in the different zones of the furnace are controlled by adjusting the heat input of the heating means in at least one of the zones to cause the temperatures in each of the zones where changes in temperature are required to approach desired temperatures. The adjustment is made in response to the last sensed temperature in a time period for each of the zones and in response to at least some of the temperatures sensed and the heat inputs measured in each of zones during that time period so as to compensate for thermal lags within the furnace and the effect of heat input in any one of the zones on the temperatures in other of the zones. However, difficulties were experienced in trying to adapt that system to forehearth control of the type disclosed in this application.
A system to control the temperatures in a forehearth must account for at least two important characteristics of the forehearth. First, there exists a thermal lag between a change in the heat input of a set of burners and changes in the temperature of the stream of molten glass in the associated forehearth zone. In addition, the temperature of the stream of glass in one zone of the forehearth will affect future temperatures in the glass streams of successive forehearth zones as the glass continues its flow to the producing devices. It is an object of the present invention to overcome the problems associated with the control techniques of the prior art by accounting for both thermal lags in the forehearth glass streams and the movement of the molten glass as it travels from the forehearth inlet through the forehearth channels to the producing devices.
In this invention, accurate control of molten glass temperatures in the streams of forehearth assemblies is achieved by dividing the forehearth into zones or regions wherein each zone is provided with a means for heating the molten glass within the zone, a means for controlling the amount of heat provided by the heating means, and a means for sensing at least one temperature in each zone. To implement the control of the molten glass temperature in a forehearth zone, the current temperature of the molten glass for such zone and the current atmospheric temperature for such zone are sensed and recorded and the current temperature of the molten glass in the immediately preceding zone is sensed. If the temperature of the molten glass in such zone is not within an acceptable range, such as 1.degree. F., of the desired molten glass temperature, the heat input of the heating means for the corresponding forehearth zone is adjusted to cause the temperature in said zone to approach the desired molten glass setpoint temperature. This adjustment is made in response to the last sensed temperature in said zone, the history of the molten glass temperatures in such zone, the atmospheric temperature in said zone and the history of the atmospheric temperatures in such zone, and the molten glass temperature in the preceding forehearth zone so as to compensate for thermal lags within the molten streams of the forehearth assembly and the movement of the molten glass as it travels to the producing devices.